Ice Ages: Why North America Is Key to Their Coming and Going

Ice Ages: Why North America Is Key to Their Coming and Going

Article excerpt

For the last 900,000 years, mile-thick ice sheets have waxed and
waned in the Northern Hemisphere with remarkable regularity -
building over periods of about 100,000 years and retreating in the
space of only a few thousand years, only to repeat the cycle.

Now, a team of scientists from Japan, the US, and Switzerland
suggests that the North American continent is the breeding ground
for these cycles. It's a region where climate and the ice's effect
on the Earth's crust play off each other to draw out the length of a
glacial cycle triggered by changes in solar radiation that come with
changes in Earth's orbit.

This feedback between climate and ice becomes most dramatic at
the end of the cycle, when an ice sheet that has bulldozed its way
too far south and gotten too heavy for its own good meets up with a
warming climate.

"When the ice sheets get to this huge state, they are very hard
to keep that way," says Maureen Raymo, a paleoclimatologist at
Columbia University's Lamont-Doherty Geophysical Observatory in
Palisades, N.Y., and a member of the team conducting the study.

Once a change in Earth's orbital characteristics brings on the
next warming event, "the whole system just goes into catastrophic
collapse," she explains. "It melts back a little, seas start to flow
into the depressed crust, this floats the ice and melts it from
below."

In a tiny fraction of the time it takes to build continental-
scale ice sheets, the sheets retreat to high-latitude havens atop
Greenland and the northern reaches of the Canadian archipelago.

The study, led by Ayako Abe-Ouchi, a climate scientist at the
University of Tokyo and the National Institute of Polar Research,
resulted from a unique approach to modeling ice ages.

The study provides "good insights that clearly advance our
understanding" of ice ages, notes Penn State University glaciologist
Richard Alley, who was not involved in the study, in an e-mail. It
also confirms aspects of ice ages that researchers have well in
hand, he says.

The approach linked individual atmosphere, crust, and ice models
in a way that needed only information on the amount of sunlight
reaching Earth to generate ice-sheet behavior over the past 400,000
years that geologists have gleaned from more than a century of field
studies.

Changes in the amount of solar radiation striking Earth come with
changes in Earth's orbit occurring at intervals of 41,000, 23,000,
and 19,000 years.

The study reaffirms that changes in the amount of summer sunlight
striking northern high latitudes sets the process in motion. Indeed,
changes to the shape of Earth's orbit over time, as well as long-
term changes in the orientation of its axis, and their impact on
solar radiation at high northern latitudes were the most significant
astronomical influences in the team's simulations.

The team also weighed the relative contributions of changes in
atmospheric concentrations of carbon dioxide, a greenhouse gas, to
the 100,000-year glacial cycles.

While carbon dioxide decreased as the ice sheet expanded and
cooled the climate and increased again as the climate warmed, CO2
levels did not determine the overall sequence of events during each
100,000-year cycle, the researchers concluded. …